The two-dimensional (2D) material means a material having a layered structure and its representative examples include graphene, transition metal dichalcogenide, etc. Further, a two-dimensional material with a thickness that is determinant to the physical/chemical properties of the material in contrast to bulkiness is referred to as “two-dimensional thin film”.
A variety of researches show that graphene is a promising material as a substitute for the existing materials used in the electronic devices. Despite its excellent properties in terms of high electron mobility, elasticity, thermal conductivity, and ductility, graphene is not suitable for transistors and optical devices due to its problem that it has a lack of band gap (0 eV for pure graphene). In contrast, transition metal dichalcogenides, such as molybdenum disulfide (MoS2) having a lamination structure in which one layer of molybdenum atoms is sandwiched between two layers of sulfur atoms through covalent bonds and Van-der-Waals attraction, have a universally tunable band gap (from an indirect band gap of 1.2 eV (bulk) to a direct band gap of 1.8 eV (monolayer)) and atmospheric stability and thus are getting the spotlight as a new two-dimensional material.
The MoS2 monolayer, first produced by the micromechanical exfoliation method similar to the approach method for the production of graphene, is counted as a promising channel material for field effect transistors (FETs). Since the publication of researches on the electrical properties of MoS2 using the dielectric screening method, studies have been performed on the various synthesis processes for MoS2, including micromechanical and chemical exfoliation, lithiation, thermolysis, and two-step thermal evaporation. With the subsequent exploitation of the sulfurization of pre-deposited molybdenum (Mo) films, the sulfurization process is verified as an appropriate synthesis method for large-area MoS2. However, the MoS2 produced from the sulfurization of pre-deposited molybdenum (Mo), relative to the exfoliated MoS2, has non-uniformity and low dielectric effect mobility and occasionally grows in the vertical direction on the substrate due to incomplete bonding between the pre-deposited Mo and sulfur (S). The chemical vapor deposition (CVD) is a well-known growth method for large-area MoS2. Therefore, the CVD method using sulfur powder and molybdenum oxysulfide (MoO3-x) reduced from molybdenum trioxide (MoO3) can be used as a very effective method of growing MoS2 atomic layers on a dielectric substrate [Lee, Y. H. et al., “Synthesis of large-area MoS2 atomic layers with chemical vapor deposition”, Adv. Mater. 24, 2320-2325 (2012)]. Studies have also been made on the production of large-area MoS2 with high quality that is of a larger crystal size or controllable in terms of the number of layers.
There has ever been no report on the appropriate method for the growth of MoS2 at low temperatures of 500° C. or less or below 400° C., so sulfurization of MoO3-x at high temperatures ranging form 550° C. to 850° C. is still on demand. Despite some studies made on the use of molybdenum pentachloride (MoCl5) or molybdenum hexacarbonyl [Mo(CO)6] as a new precursor for the growth of MoS2, the synthesis at low temperatures tends to induce the creation of MoS2 having a three-dimensional structure on the substrate. Typically, the higher temperature leads to the use of fewer nuclei, the larger surface diffusion length, and the more effective desorption of volatile substances and thus makes the growth of high-quality films easier. In contrast, the lower temperature results in the smaller critical radius for the nucleation and the smaller surface diffusion length, so it is challenging to the growth of a high-quality film, particularly a monolayer.
There are two representative properties determinant to the quality of the two-dimensional thin films: grain size and uniformity. Conventionally, the two-dimensional thin films with a larger grain size are considered to have the higher quality. According to a recent research [K. Kang, X. Xie, L. Huang, Y. Han, P. Y. Huang, K. F. Mak, C.-J. Kim, D. Muller & J. Park, “High-mobility three-atom-thick semiconducting films with wafer-scale homogeneity”, Nature, 520, 656-660, 2015], a highly uniform two-dimensional thin film produced by the CVD method has material properties three to four times more excellent than those of the second-dimensional thin film prepared by the exfoliation method and known to have a highest quality. Hence, the synthesis of two-dimensional thin films with high uniformity is emerging as a core technology deciding the quality of the two-dimensional thin films.
The current MoS2 production methods require the high temperature condition of at least 550° C. in order to synthesize MoS2 films with a uniform thickness. Further, there has never been yet reported any appropriate synthesis method for MoS2 with a uniform thickness at low temperatures of 550° C. or below. Under the high temperature condition, the mobility of MoS2 molecules on the surface of the substrate (typically, dielectric substrate) is so high that a monolayer (i.e., a first layer) of MoS2 rather than a bilayer (i.e., a second layer) is produced uniformly in a large area (at least 8 in.) on the substrate. Under the low temperature condition, the MoS2 molecules have such an extremely low mobility on the surface of the substrate to grow into crystals larger than a defined grain size, incurring the creation of a bilayer of MoS2 before the large-area substrate is entirely coated with a monolayer of MoS2. This induces the formation of MoS2 having a three-dimensional (3D) structure on the substrate.
More specifically, the mobility of molecules on the surface of the substrate is expressed in terms of the diffusion length according to the Einstein's relation (Equation 1). When the synthesis temperature is extremely low, the surface diffusion length is so small as to cause evaporation of the molecules from the surface before the formation of molecular bondings. Further, the Van-der-Waals attraction prevents the molecules adsorbed onto the monolayer from moving towards the edge sides of the monolayer and provides nucleation sites for the growth a bilayer to synthesize a thin film typically in the Volmer-Weber or Stranski-Krastanow growth mode (Refer to FIG. 14). Hence, the synthesis of a monolayer thin film with high uniformity is generally performed at high temperature. In other words, under the high temperature synthesis condition, a thin film is synthesized in the Frank-Van der Merve growth mode due to the high mobility of molecules on the surface of the substrate, so the whole substrate can be coated with a monolayer thin film. But, under the low temperature synthesis condition, the mobility of molecules on the surface of the substrate is so low as to induce the synthesis of a thin film in the Volmer-Weber or Stranski-Krastanow growth mode, resulting in a failure to coat the whole substrate with a monolayer thin film.
                              λ          =                                    λ              0                        ⁢                          exp              ⁡                              (                                                                            E                      a                                        -                                          E                      d                                                                            2                    ⁢                                                                                  ⁢                                          K                      B                                        ⁢                    T                                                  )                                                    ⁢                                  ⁢                  λ          ⁢                      :                    ⁢                                          ⁢          Diffusion          ⁢                                          ⁢          length                ,                                  ⁢                  T          ⁢                      :                    ⁢                                          ⁢          Temperature                ,                                  ⁢                              λ            0                    ⁢                      :                    ⁢                                          ⁢          Pre          ⁢                      -                    ⁢          exponetial          ⁢                                          ⁢          factor                ,                                  ⁢                              E            a                    ⁢                      :                    ⁢                                          ⁢          Adsorption          ⁢                                          ⁢          energy                ,                                  ⁢                              E            d                    ⁢                      :                    ⁢                                          ⁢          Diffusion          ⁢                                          ⁢          barrier                ,                                  ⁢                              K            B                    ⁢                      :                    ⁢                                          ⁢          Boltzmann          ⁢                                          ⁢          constant                                    [                  Equation          ⁢                                          ⁢          1                ]            
Accordingly, there is a demand for the development of a preparation method for a two-dimensional thin film with high uniformity, more specifically, a highly uniform two-dimensional transition metal dichalcogenide thin film on a substrate having a large area of at least 8 in. even in the low temperature condition of 500° C. or below.